The present invention relates to the field of the duplication of extensive planar objects and more particularly to an optical printing device for planar objects having unity magnification, being more specifically applicable to the duplication of high resolution masks.
The generation of a wave front coupled with an object wave front makes it possible to form a real image of the object. This generation can be obtained by holography. On the basis of the hologram of an object recorded in a holographic support and by illuminating the hologram with a reading beam it is possible to obtain an image of the object illuminated during the recording. If the reading beam has the opposite transmission direction to that of the reference beam during restoration, a real image of the object is formed and this image can be recorded on a photosensitive support placed in an image plane. As the beams are coherent, the restored wave front is stigmatic for all points of the image. However, due to the coherent nature of the object beam and reference beam, the restored image is subject to laser speckle, which significantly reduces the said signal to noise ratio in the restored image. Thus, the image appears fuzzy through light points, this modulation of the image being superimposed on the surface image. Account must be taken of this phenomenon in any application necessitating the projection of a real image onto a photosensitive surface. There are several possible solutions for limiting the effects thereof in the restored image.
A first solution consists of forming a relatively large hologram in order to obtain in the projected image a greater resolution than that required for the recording of this image on the photosensitive support. Thus, if the size of the light grains formed is small compared with the smallest dimension of the recorded pattern, the pattern is recorded with an average noise corresponding to the average intensity over several grains. Such a solution is not sufficient for the reproduction of large high resolution masks.
The averaging action can also be obtained by juxtaposing a large number of small holograms in a conventional, non-obliteratable holographic support which is able to restore an image after developing. In this case, each hologram restores a complete real image of the object on the photosensitive surface and the superimposing of these images containing the same information but with different noise levels leads to an averaging of the noise on the surface. However, in order to improve the signal to noise ratio in a by a factor n, the number of superimposed images must be n.sup.2, which necessitates a very large holographic recording support compared with the object to be reproduced.